WO2022175570A1

WO2022175570A1 - Method for obtaining a plasticised material, the plasticised material obtained and use thereof

Info

Publication number: WO2022175570A1
Application number: PCT/ES2021/070747
Authority: WO
Inventors: Ignacio Martín Gullón; Juan Carlos GARCÍA QUESADA; Mercedes GARCÍA MONTALBÁN; Daniel DOMENE LÓPEZ
Original assignee: Universidad De Alicante
Priority date: 2021-02-19
Filing date: 2021-10-14
Publication date: 2022-08-25
Also published as: EP4296311A1; US20240110021A1; ES2897213B2; ES2897213A1

Abstract

The present invention relates to a method for obtaining an improved plasticised material resulting from two differentiated steps: an esterification reaction for synthesising a polycarboxylic acid ester oligomer in the presence of excess polyol and a catalyst, and a second step of gelatinisation and plasticisation at temperatures above 90ºC involving a starch mixed with water and with the polycarboxylic acid ester oligomer with excess polyol obtained in the previous step, and a coadjuvant. Advantageously, the catalyst used in the esterification reaction is the same as the coadjuvant used in the gelatinisation and plasticisation reaction. The present invention also relates to the improved plasticised material obtained by means of the described method, which has a high stability and a low migration rate compared to known plasticised materials and the use thereof.

Description

PROCEDURE FOR OBTAINING A PLASTICIZED MATERIAL. THE MATERIAL

PLASTICIZED OBTAINED AND ITS USE

TECHNICAL SECTOR

The present invention relates to a plasticized material which is based on thermoplastic starch and which, advantageously, has a low plasticizer migration, so it is intended for use as flexible or rigid packaging.

Another object of the present invention is the procedure used to obtain it, which is necessarily carried out in two different and independent stages: an esterification reaction stage and a second stage of gelatinization and plasticization at temperatures above 100°C.

BACKGROUND OF THE INVENTION

Known thermoplastic starch-based polymers exhibit high plasticizer migration and thus exhibit little temporal stability of their properties.

Known thermoplastic starch formulations require gelatinizing and plasticizing the starch in the presence of plasticizers, which is typically water (which is largely removed later) and another higher boiling point plasticizer. In conventional formulations, glycerol and other low molecular weight polyols are usually used as a plasticizer.

Plasticized starch, surrounded by plasticizer molecules, has a high tendency to retrogradation, that is, it partially recovers its original ordered structure, which produces a decrease in its properties.

Given the low molecular weight of glycerol, there is little steric hindrance and the starch chains are rearranged in relatively short times, causing the migration of glycerol towards the surface of the starch material and thus decreasing the stability of the physical and mechanical properties of said material. , becoming very brittle.

At the same time, thermoplastic starch formulations have high hydrophilicity and their properties decrease a lot in environments with high relative humidity. Therefore, thermoplastic starch has not been implemented because it is not very durable and not at all stable. Likewise, thermoplastic starch formulations are difficult to process using conventional extrusion machinery and are usually co-processed with other polymers, obtaining greater stability and processability, but depending on which one is used, it will have a different degree of biodegradability and environmental impact. Polyvinyl alcohol (PVA) is a highly biodegradable hydrophilic polymer and, co-processed with thermoplastic starch, has a synergistic effect forming a compound, and other adjuvants such as zinc stearate may also intervene. However, these compounds (or blends) based on thermoplastic starch still lack high temporary stability and resistance to environmental humidity.

In this technical field, patent document no. US20070072988A1 which discloses methods for the manufacture and synthesis of plasticizers based on tricarboxylic acid esters. However, the document does not mention starch, although tests are carried out on the permanence of the plasticizer.

The document no. US 2011/0046283A1 discloses citric acid mixtures and their uses. This document uses mixtures of esters obtained from citric acid as PVC plasticizers with the aim of replacing conventional plasticizers (DEHP or DINP) with others of less toxicity. No migration studies are performed in the document, nor is starch mentioned.

On the other hand, document no. US4931583A describes the synthesis of citrate esters that serve as PVC plasticizers and that have less toxicity. Starch is not mentioned in the document.

The document no. US 2019/0055376A1 discloses plasticizer compositions containing polymeric dicarboxylic acid esters and dialkyl esters of phthalic acid. Certainly, the patent document does not mention starch, but it is applied to thermoplastic polymers in general, particularly in the case of PVC.

Finally, two scientific articles related to the object of the present invention are cited. Specifically, the article by Halpern, JM, Urbanski, R., Weinstock, AK, Iwig, DF, Mathers, RT, & von Recum, HA (2014) entitled "A biodegradable thermoset polymer made by esterification of citric acid and glycerol” published by the Journal of Biomedical Materials Research (Part A, 102(5), 1467-1477). This article details the esterification reaction of citric acid and glycerol. However, the obtained polyester is not applied as a starch plasticizer.

And the article titled “PVC materials without migration obtained by Chemical modification of azide-functionalized PVC and triethyl citrate plasticizer” by the authors Jia, P., Hu, L, Feng, G., Bo, C., Zhang, M., & Zhou, Y. published in 2017 by “Materials Chemistry and Physics” (190, 25-30). In this article, esters are obtained from citric acid and applied as PVC plasticizers, but not applied to starch.

That is why the applicant of the present patent application detects the need to develop a process that allows obtaining a plasticized material based on thermoplastic starch that is stable and durable over time.

DESCRIPTION OF THE INVENTION

The object of the invention of this patent is the process for obtaining an improved plasticized material, which is made up of at least two different stages that are detailed below:

First stage: an esterification reaction between at least one dicarboxylic and/or tricarboxylic acid and a polyol, using a zinc-based catalyst or a solid zeolitic catalyst, such that the hydroxyls of the polyol are in excess of the carboxyls of the dicarboxylic and/or tricarboxylic acid, obtaining an oligomer of polycarboxylic acid esters in the presence of excess polyol and catalyst. Second stage: a gelatinization and plasticization reaction at temperatures above 90°C of the starch mixed with water and with the oligomer of polycarboxylic acid esters with excess polyol, obtained from the previous stage, and an adjuvant.

Advantageously, the catalyst used in the esterification reaction is the same component as the adjuvant used in the gelatinization and plasticization reaction. In this way, the component in question acts as a catalyst in the first stage, while the remaining excess acts as an adjuvant in the second stage. On the other hand, the oligomer of polycarboxylic acid esters (plasticizer) obtained in the first stage has a mobility of the chains that make it up less than the mobility of the chains of the thermoplastic starch that intervenes in the second stage.

Advantageously, carrying out the esterification stage separately from the gelatinization and plasticization stage of the starch makes it possible to synthesize in the first stage in a controlled and catalyzed manner the oligomer of esters of branched polycarboxylic acids with a low number of units - mainly when the polyol is in stoichiometric excess - and few carboxyl groups remain unreacted, thereby minimizing or mitigating a possible crosslinking that could occur when the aforementioned oligomer is added to starch during the gelatinization and plasticization stage.

In addition, the oligomer of polycarboxylic acid esters obtained from the first stage of the process will have compatibility as a plasticizer for starch-based materials, as it has an excess of hydroxyl groups, plus the unreacted excess polyol, favoring the reduction of migration, giving rise to an improved plasticized material with more stable mechanical properties over time.

Well, as stated above, a first stage of esterification and synthesis of the oligomers of esters of polycarboxylic acids with polyols is carried out.

For this, the synthesis of the oligomers is carried out in a reactor by means of an esterification reaction between one or more dicarboxylic and/or tricarboxylic acids and a polyol using zinc-based catalysts or other solid zeolitic catalysts, such as the commercial Amberlyst 15 ® catalyst.

Preferably, the dicarboxylic and/or tricarboxylic acids used in the esterification reaction are citric acid, malic acid and/or tartaric acid, among others. While the polyol used in the esterification reaction and in the gelatinization and plasticization reaction has several hydroxyl groups, glycerol or sorbitol, among others, are preferably used.

On the other hand, the zinc-based catalyst used in the esterification reaction is zinc stearate, where it provides lubricating properties and will initially favor instances the subsequent gelatinization process.

Preferably the mixture of acid and polyol should be such that the hydroxyls are in excess of the carboxyls, so that all of the acid molecules have reacted (although perhaps not all of the carboxyls). At the end of the first stage, a viscous liquid mixture is obtained containing the polycarboxylic acid ester oligomers in the presence of excess or unreacted polyol, in the form of a sol-gel, and the catalyst.

After completing the first stage of the procedure for obtaining improved plasticized material, the second stage is carried out, which concerns gelatinization and plasticization at temperatures above 90°C.

In the second stage, a mixture of water, the oligomer of polycarboxylic acid esters, polyol and the adjuvant is prepared as a gelatinization and plasticization medium, which are mixed with the starch. It should be noted that the polycarboxylic acid ester oligomer is an element resulting from the first stage, while the polyol and the auxiliary are compounds that were present in excess in the first stage or esterification stage.

During the gelatinization stage, carried out at temperatures above 90°C, the oligomer or oligoester of medium molecular weight, but highly branched, easily penetrates between the gelatinized starch chains.

At the end of the gelatinization stage, much of the water has been eliminated due to the effect of temperature and the shear applied in the process. In this way, the plasticized material (thermoplastic starch), once cooled, has a lower mobility of the chains and the plasticizer used (polycarboxylic acid ester oligomer), due to steric hindrances, also presents a lower migration towards the surface. of the material compared to other plasticized thermoplastic starches, for example, with glycerin.

Optionally, a thermoplastic polymer PVA (polyvinyl alcohol) is involved in the gelatinization and plasticization reaction, acting as a high molecular weight plasticizer, which together with the starch forms a blend that is mixed with water and with the oligomer of polycarboxylic acid esters. with excess polyol and an adjuvant. In this sense, with the introduction of plasticizers with a higher molecular weight and steric hindrance -as is the case with the thermoplastic PVA-, a slight bond is added to the polymer molecules, and their migration is reduced, thus controlling the starch retrogradation. This allows the mechanical properties to be much more stable over time, in addition to presenting much higher values, without detriment to the fact that the plasticized material obtained from the procedure will continue to be biodegradable, mainly when it comes into contact with water.

If starch is involved in the gelatinization and plasticization stage, without the presence of another thermoplastic, the material must be shaped before proceeding with cooling (for example, by means of cast film or thermo-blown film).

If the starch is involved in the gelatinization and plasticization stage together with another thermoplastic, such as the PVA thermoplastic, cooling occurs directly and it can be reprocessed later as a melt.

The detailed procedure is novel and innovative because:

The use of a plasticizer in the gelatinization and plasticization stage based on an oligomer of esters of polycarboxylic acids and polyols, which has a higher molecular weight and large steric volume compared to other common plasticizers, controls the retrogradation of thermoplastic starch after elimination. of the water.

The presence of the zinc-based catalyst or the solid zeolitic catalyst mixed with the oligomer - obtained in the first stage - promotes the esterification reaction between free carboxylic groups of the oligomer with starch hydroxyls, forming a slight crosslinking, which also contributes, together with the steric volume, to the slowing down of the mobility of this plasticizer with respect to the starch chains, greatly limiting the migration of the plasticizer with respect to when only glycerol is used.

The use of zinc stearate as a catalyst in the esterification reaction and as an adjuvant in the gelatinization and plasticization reaction provides a triple function in the process for obtaining the improved plasticized material.

Advantageously, the improved plasticized material obtained in the gelatinization and plasticization stage presents very important mechanical increases with respect to its counterparts that use only water and glycerol. On the other hand, if mixtures of starch and the thermoplastic polymer PVA are used in the gelatinization and plasticization stage, the PVA intervenes in a percentage of between 40% and 60% - preferably 50% The use of water and mixture of the oligomer of polycarboxylic acid esters obtained in the previous stage of esterification allow obtaining a plasticized material with improved tensile mechanical properties with increases in the maximum value of maximum tension of 100% with respect to the base material (plasticized only using glycerol) , increases in the elastic modulus of more than 150% (above 20 MPa) and elongations at break of more than 500%.

Likewise, the present invention recommends a thermoplastic starch compound - plasticized material - that incorporates a previously synthesized plasticizing material based on polyol polyester and polycarboxylic acid, so that the thermoplastic starch compound can control the retrogradation of starch and reduce their rate of migration.

It is necessary to point out that the improved plasticized material obtained reduces the plasticizer migration index from 16% in the base material to migration index values of less than 3% in the materials using the aster oligomer mixture, as has been confirmed. in the tests that will be shown in the section on the preferred embodiment of the invention.

Likewise, the use of the plasticized material obtained according to the procedure detailed above as flexible packaging is an object of the invention. Being especially interesting for its use in feed bags, due to its low degree of migration.

On the other hand, the use of the plasticized material obtained according to the procedure detailed above as rigid packaging, for example, for its materialization in containers that can be sealed hermetically, is also an object of the invention.

Therefore, it is necessary to highlight that the improved plasticized material obtained from the previously detailed procedure has a migration rate with respect to time of less than 10%, while its modulus of elasticity is at least 10 MPa. BRIEF DESCRIPTION OF THE DRAWINGS

To complement the description that will be made below, and in order to help a better understanding of the characteristics of the invention according to the preferred examples of its practical embodiment, a set of figures is attached as an integral part of said description. where, by way of illustration and not limitation, the following has been represented:

Figure 1.- Shows a graphic representation of the yield of the polymerization reaction, monitored by the carboxylic groups reacted against time according to example 1.

Figure 2.- Shows a graphical representation of the derivative of the mass variation with respect to temperature for pure compounds (citric acid and glycerol) and binary mixtures prepared according to example 1.

Figure 3.- Shows a graphic representation for the samples of example 2 of the performance of the polymerization reaction with different carboxylic acids and glycerol, varying the excess of glycerol, monitored by the percentage of carboxylic groups reacted against time.

Figure 4 Shows a graphic representation of the mechanical properties of the samples of example 3.

Figure 5.- Shows a graphical representation of the migration index with respect to time of the samples of example 3.

PREFERRED EMBODIMENT OF THE INVENTION

In this section, three examples of implementation of the invention are provided that allow verifying the arguments cited above that lead to confirming the advantageous properties that the improved plasticized material obtained by means of the developed procedure presents.

Specifically, the examples carried out are the following: Example 1: Synthesis of the oligomer from citric acid and glycerol. Effect of reaction time.

Example 2: Synthesis of the oligomer using citric acid, malic acid and tartaric acid, and glycerol demonstrating the influence of excess glycerol on the conversion yield of carboxylic acids using zinc stearate as catalyst.

Example 3: Formulations of thermoplastic starch and polyvinyl alcohol by melt blending. Effect of the use of polyester oligomers and glycerol on the properties.

Specifically, in example 1 it is carried out according to an embodiment of the first stage of the process of the invention, the objective being the synthesis of the oligomer of polycarboxylic acid esters from citric acid -because it is a tricarboxylic acid- and glycerol -because it is a polyol- in order to check the reaction time.

For the preparation of example 1, two equal mixtures of glycerol and citric acid are prepared, where the glycerol has a 60% molar excess (70.6 mmol of citric acid, 113.1 mmol of glycerol, for a total of 24 g).

Amberlist-15 ® (1 mol% of the total) (type B samples) and 1.13 g of zinc stearate (type A samples) are added as catalyst to each mixture. They are vigorously mixed and placed in an oven at 110°C for the formation of the oligomer at different times: 0.2, 0.5, 1, 2, 5, 10, 24, 48 and 72 hours. Likewise, a mixture without catalyst was prepared as a control, and the treatment was carried out in an oven at 110°C for a single time of 10 hours.

The yield of the esterification reaction was evaluated by titration of free carboxylic groups with sodium hydroxide. For this, 2 grams of mixture reacted at different times (type A and type B samples) were introduced in 500 ml_ of deionized water and vigorously dispersed until complete homogeneity.

Finally, aliquots were titrated with 1M NaOH using phenolphthalein solution as indicator. The extent of the polymerized oligomer structure was monitored by differential thermal analysis (DTG), using a NETZSCH TGA/STA 449 F5 Jupiter equipment.

Figure 1 represents the reaction time (expressed in hours) on the abscissa axis and the reaction yield (expressed as a degree of conversion in %) on the ordinate axis. Thus, figure 1 shows the yield of the esterification reaction for the type samples A and type B samples, measured as a function of the number of reacted carboxylic groups.

Figure 1 shows that final values of 70 and 60% yield are reached when Amberlyst-15 ® (type B samples) and zinc stearate (type A samples) are used, respectively, although these values are already practically reached for reaction times of 10 hours.

Since 1 molecule of citric acid has 3 carboxylic groups, it can be confirmed with high probability that all citric acid is reacted, and due to steric reasons and action of each heterogeneous catalyst, unreacted carboxylic groups remain in the oligomers.

The new mixtures form a single phase and are much more viscous, and have an excess of glycerol. For the mixture without catalyst as a control, the polymerization was carried out at 10 hours, with a yield of 39%.

In order to evaluate the behavior of the polycarboxylic acid ester oligomer with the catalysts of the invention, a differential thermal analysis is performed.

For this, a comparison of the following compounds is made: pure citric acid (sample C), pure glycerol (sample D), mixtures with a proportion of 57% citric acid polymerized with zinc stearate (sample E) for 10 hours , and mixtures with a proportion of 57% in citric acid polymerized with Amberlyst-15 ® (sample F) for 10 hours.

Figure 2 shows the representation of the differential thermal analysis carried out, where the derivative of the mass loss with respect to temperature (DTGA expressed in %/°C) is represented against temperature (expressed in °C) for the mentioned compounds (sample C, sample D, sample E, sample F).

Thus, in figure 2 it can be seen that pure glycerol (sample D) evaporates completely (without decomposition) at 200°C, while pure citric acid (sample C) partially decomposes from 175°C, finishing completely decomposing. also at 200°C.

On the other hand, Figure 2 also clearly shows that the polycarboxylic acid ester oligomer reacted for 10 hours at 110°C with zinc stearate (sample E) and in the polycarboxylic acid ester oligomer reacted for 10 hours at 110°C with Amberlyst-15 ® (sample F), some glycerol remains, but most of the material has greater thermal stability, presenting a broad peak maximum decomposition at 270°C.

Therefore, from example 1 it is concluded that the material obtained from the polycarboxylic acid ester oligomer reacted with zinc stearate (sample E) and the polycarboxylic acid ester oligomer reacted with Amberlyst-15 ® (sample F) decompose at very high temperatures. high (270°C), so it can be concluded that these oligomers have a relevant thermal stability.

In example 2, the synthesis of the oligomer is carried out using citric acid, malic acid and tartaric acid and glycerol in order to verify the influence of excess glycerol on the conversion yield of carboxylic acids using zinc stearate as catalyst.

Example 2 reproduces some specific embodiments of the first stage of the process for obtaining the improved plasticized material of the present invention. Thus, example 2 starts from binary acid-glycerol mixtures of 24 total grams, which contain different mass percentages of acid in the mixture: 5%, 10%, 25% and 45%.

In Figure 3, series G corresponds to the samples containing citric acid, series H corresponds to the samples containing malic acid and series I corresponds to the samples containing tartaric acid.

In addition, mixtures were prepared where the stoichiometric excess of hydroxyls versus carboxyls was 60% (st-60 mol) (in figure 3 it corresponds to the 60% bars). The 60% sample bar G corresponds to 57% by weight of the sample with citric acid. The 60% sample bar H corresponds to 48% by weight of the sample with malic acid. Bar I of the 60% sample corresponds to 50.5% by weight of the sample with tartaric acid.

Additionally, J samples are prepared that correspond to a control for each of the acids, in which the proportion of the acid is 5% and in which the esterification reaction has not been carried out. To all these mixtures of Example 2, 1% molar of zinc stearate is added, which will act as a catalyst. They are vigorously mixed and placed in an oven at 110°C for 10 hours for the formation of the polycarboxylic acid ester oligomer at different times.

The yield of the reaction was carried out by titration of free carboxylic groups with sodium hydroxide. To do this, 2 grams of the reacted mixture were introduced into 500 ml_ of deionized water and vigorously dispersed until complete homogeneity. Finally, aliquots were titrated with 1M NaOH using phenolphthalein solution as indicator.

Figure 3 shows the results of the degree of conversion of the polymerization monitored by the molar % of reacted carboxylic groups with respect to the initial total for the different mixtures prepared at 5%, 10%, 25% and 45%.

From the results obtained and represented in Figure 3, it should be noted that the mere physical mixture of acids and glycerol does not produce a reaction (sample J), although it does produce a possible masking of 5%. On the other hand, the excess of glycerol does not seem to affect the overall yield to a great extent either, that is, although there is a lot of excess of glycerol, practically the same percentage of free carboxylic groups remains, which probably do not react for steric reasons due to the three-dimensional configuration of the glycerol. oligomer of polycarboxylic acid esters formed.

As a general trend, it is observed that for the sample with a 60% stoichiometric molar excess in glycerol, the proportion of reacted carboxylic groups decreases slightly for citric acid and malic acid.

Another general trend observed is that tartaric acid (bicarboxylic and bihydroxylic) has a slightly higher number of functional groups than citric acid (tricarboxylic) and malic acid (bicarboxylic, but slightly less polar). Although the three-dimensional structure of the oligomers will be different.

Finally, example 3 is carried out in relation to the formulations of thermoplastic starch and polyvinyl alcohol by melt blending in order to analyze the effect of the use of polyester oligomers and glycerol (obtained in the first stage of the process of the invention) on the properties of the plasticized material obtained.

In example 3, 5 different samples of thermoplastic starch and polyvinyl alcohol (PVA) were prepared with oligomer mixtures of polymerized esters of citric acid and glycerol, catalyzed with zinc stearate (all in stoichiometric excess of glycerol), plus water as a medium. gelatinization. Two control samples were also prepared.

The samples were prepared according to the following table:

The potato starch supplied by Sigma Aldrich was used. However, the starch used is not limiting and we can use different starches from different botanical sources: potato, wheat, corn or rice; and with different previous treatments (pre-gelatinized, "waxy starches", etc.) obtaining equivalent results with the technical advantages they offer.

Mowiol 2098 type supplied by Sigma-Aldrich was used as PVA. However, the PVA used is not limiting and we can use PVA such as: Mowiol 10-98, Mowiol 18-88, Mowiol 29-99 and other types of commercial PVA with different viscosity and degree of hydrolysis, obtaining equivalent results with the advantages techniques they offer.

Zinc stearate was supplied by Sigma-Aldrich.

Mixtures of 24 grams of ester oligomer were used, catalyzed by zinc stearate (1% molar) at 110°C for 10 hours (procedure described in example 1), starting from binary mixtures of citric acid and glycerol with proportions of citric acid of 5, 10, 25, 45 and 57% (with nomenclature: sample 5%, sample 10%, sample 25%, sample 45% and sample 57%).

For the two control samples were used:

Control sample 1: mixture of 24 g of citric acid plus 57% glycerol to which added to the rest of the components without prior reaction (sample J).

Control sample 2: mixture of 24 g of citric acid plus 57% glycerol that were treated at 110°C and 10 h without catalyst (sample K).

The components were introduced into a bottle, and the powder mixtures were mixed well with the liquid ones, leaving it to stand for one hour. Next, they were introduced into a Haake Polylab Qc plastograph, with a 50 mL chamber and corrotating rollers according to the following process: a) 5 minutes at 110°C and spindle speed at 50 rpm, and b) 5 minutes at 110 °C and spindle speed at 100 rpm.

Finally, 1 mm thick films (thin sheets) were prepared in a hot plate press at 160°C, according to the following procedure: a) 5 minutes without pressure, to favor progressive melting, b) 10 minutes at 7 ton-force. It is then flash cooled to room temperature by conduction.

The tensile mechanical characterization of the samples was carried out following the ASTM D-882 standard, obtaining specimens of the 1 mm thick sheet by means of a die. An Instron 3344 multi-test machine was used. From this test, the modulus of elasticity or Young's (E), the maximum breaking strength (a _max ), and the elongation at break were obtained.

(stear)

Figure 4 shows the tensile mechanical properties for the samples under study, namely: the 5 samples with a plasticizer polymerized with zinc stearate, different proportions of citric acid and glycerol (5% sample, 10% sample, 25% sample). , sample 45% and sample 57%), and the two control samples: one with a mixture of unreacted plasticizers (sample J) and another reacted mixture without catalyst (sample K).

In Figure 4, the values of the modulus of elasticity/Young (bars L) and maximum resistance to breakage (bars M) (both expressed in MPa) are found on the main ordinate axis, and the elongation at break (bars N) (expressed in %) is displayed on the secondary axis.

Thus, in Figure 4 it can be seen that the lowest modulus value is obtained in the control sample (sample J), and as the proportion of citric acid increases, the modulus increases more, indicating less mobility of the reacted oligomer. The highest modulus value is obtained with the 45% sample, with a value more than 15 times higher than the unreacted control sample. The 57% sample also has a high modulus, as well as the highest maximum strength of all the samples, with an elongation at break of 600%, which directly implies a high toughness value. In fact, using zinc stearate as a catalyst in the oligomer confers much superior mechanical properties with respect to the second control sample (K) which does not use a catalyst in the polymerization.

In addition to the tensile mechanical characterization tests - within example 3 - migration tests were carried out on the samples obtained, the results of which are shown in Figure 5.

To determine the migration of the plasticizer used in the biopolymer or improved plasticized material obtained from the process claimed in the present invention, circular specimens of the 1 mm thick sheet with a diameter of 7 mm were punched.

The aforementioned circular specimens were placed between two petri dishes on absorbent paper, applying a pressure of 16.5 kPa at 60 °C for different times (1, 2, 5 and 7 days).

To determine the evolution of the migration of the plasticizer to the surface, after each of these times has passed, the calculation of the relative reduction between the initial and final weight, divided by the initial weight, has been carried out.

Therefore, figure 5 shows the evolution of the migration index (expressed in %) for the different samples (5% sample, 10% sample, 25% sample, 45% sample and 57% sample) and the plasticizer control sample. with 5% citric acid which was not subjected to polymerization (sample J). The formulations represented in Figure 5 are mixtures of thermoplastic starch and PVA in a gravimetric ratio of 1: 1 with 60 phr of plasticizer based on citric acid and glycerol previously catalytically polymerized with 1 mol% zinc stearate (with the exception of sample J ), at 110°C for 10 hours

As can be seen in Figure 5, the sample with the highest migration percentage value is control sample J (using unreacted citric acid and glycerol), while All the samples that use polymerized plasticizer show less migration (sample 5%, sample 10%, sample 25%, sample 45% and sample 57%) compared to the time expressed in days. Sample 45% and sample 57% - which contain the highest concentration of oligomer of polycarboxylic acid esters and less excess of glycerol - are the ones with the lowest migration index, around 3%, while sample J control has a migration index of more than 10%. Therefore, the results obtained in this last test and represented in Figure 5 allow us to conclude that the improved plasticized material obtained according to the procedure detailed above has a lower migration index as mobility in its structure is impeded by the synthesis in two differentiated stages. (esterification and gelatinization).

Claims

1 a.- Procedure ^for obtaining a plasticized material comprising the following stages:

Esterification reaction between at least one dicarboxylic and/or tricarboxylic acid and a polyol, using a zinc-based catalyst or a solid zeolitic catalyst, such that the hydroxyls of the polyol are in excess of the carboxyls of the dicarboxylic acid and/or or tricarboxylic, obtaining an oligomer of polycarboxylic acid esters in the presence of the excess polyol and the catalyst.

Gelatinization and plasticization reaction at temperatures above 90°C of starch mixed with water and with the polycarboxylic acid ester oligomer with excess polyol obtained in the previous stage, and an adjuvant. characterized in that the catalyst used in the esterification reaction is the same as the adjuvant used in the gelatinization and plasticization reaction, and where the polycarboxylic acid ester oligomer (plasticizer) has a mobility of the chains that make it up less than the mobility of thermoplastic starch chains.

2 .- Procedure ^for obtaining a plasticized material, according ^to claim 1, characterized in that the dicarboxylic and / or tricarboxylic acids used in the esterification reaction are citric acid, malic acid and / or tartaric acid.

3 .- Procedure ^for obtaining a plasticized material, according ^to claim 1, characterized in that the polyol used in the esterification reaction and in the gelatinization and plasticization reaction has several hydroxyl groups, glycerol or sorbitol being used.

4 .- Process ^for obtaining a plasticized material, according ^to claim 1, characterized in that the zinc-based catalyst used in the esterification reaction is zinc stearate.

5 .- Procedure ^for obtaining a plasticized material, according ^to claim 1, characterized in that a PVA thermoplastic polymer is involved in the gelatinization and plasticization reaction of the starch. 6 .- Procedure ^for obtaining a plasticized material, according ^to claim 5, characterized in that the PVA thermoplastic polymer is involved in the gelatinization and plasticization reaction of starch in a percentage of between 40% and 60%. 7 .- Plasticized material obtained according ^to the procedure detailed in any of the preceding claims, characterized in that its migration rate with respect to time is less than 10%.

8 .- Plasticized material obtained according ^to claim 7, characterized ⁱⁿ that its modulus of elasticity is at least 10 MPa.

9 .- Use ^of the plasticized material obtained, according to the preceding claims, as flexible packaging, preferably in food bags. 10 .- Use ^of the plasticized material obtained, according to the preceding claims, as rigid packaging.